When the clients ask me a question and I don't have the answer, you are NOT suggested saying _______.

mm, that's a good question. But hold on for a while. Let me check and find out for you.|Sorry, I don't know. Your question doesn't concern our business.|I believe one of our people can help you. Shall I call you back 5 minutes later?|I'll check with my manager. Are you able to hold?